and that’s what we’re down to.
Today’s topic will be a chapter in the strangely believe it book. This is about literally about three or four drops at most that hit me at 5:18 PM LST while outside photographing clouds (of course!). I really didn’t think they were drops from clouds, I could not see a darn thing that could have done this overhead or downwind, I mean zilch, nada, nil zero, in my esteemed estimation. I cast them off as something due to a misuse of water by my neighbor. Definitely, it had to be some pseudo-sprinkle. Just to be sure, I looked on the top of an iron fence, and there were exactly two drops. Wow, I thought, could drops really have fallen out of the sky with virtually no cloud??? And more technically, no indication whatsoever of ice in those small Cumulus? One more check, the dusty car. It was dumbfounded to see that there was a smattering of small drops there as well! How could this be? It was the Twilight Zone for me.
Here is the sky about 10 minutes before (1), one minute before (2) five minutes after (3), and (4) comprises an “explanation”; extremely thin veils of ice crystals can clearly (he sez) be seen below the tiny cumulus clouds. Perhaps there were just too few ice crystals in those approaching clouds, and those downwind to see any frizzy bottoms due to virga in #3. I sure couldn’t see it, nor really did I see any ice elsewhere except far to the northwest to north later in the afternoon.
So how cold was it at cloud top? If you’re really into clouds and stuff, you’ll want to know, of course.
Well, there was a sounding released around 3:30 PM or so in the afternoon for the 0000 Z (Greenwich) time sounding from Tucson. The tops it indicated were but -10 C. I will now purport that that temperature is incorrect for the clouds that passed over me and are those tiny Cumulus shown in #4. Can’t happen with -10 C tops on THIS kind of day. So I reject that sounding as applicable to MY clouds! (Hmmm, nice RW- right now at 5:42 AM, yay, might even darken the pavement!) So what is the best explanation for ice in small clouds? Probably the air got slightly colder after the sounding was released, but that probably would not account for too much change, but something in the right direction of colder tops than -10 C. I would say that more important was the stunning drop in temperature aloft as you go north from Tucson being the best explanatory “culprit”. That is, it was significantly colder of us here in Catalina that it was where that sounding balloon went up. Being colder aloft would allow the tops to be a bit higher and colder, as well as being cooler just because they were to the north of the sounding. Flummoxed by this dizzying explanation? I hope not. In sum, those kinds of small clouds that produce ice were probably colder at top than about -12 C, and so we’re not talking a lot.
Summary: Colder to the north, cloud tops were a bit higher, too. Result, ICE (required for rain nearly always in AZ)!
Just recorded 0.o1 inches here at 5:52 AM! Yes, Virginia, it CAN rain in SE Arizona!
Now about dust…. Dust has been implicated as helping clouds to rain, particularly in desert environments (sweet! What an amazing thing that is, the desert soil having particles that help the clouds overhead to precipitate. Unbelievable.
There is something to that. In the olden days of the 1970s during the Colorado River Basin Pilot Project, a huge randomized cloud seeding experiment, the highest values of ice nuclei measured on the ground, anyway, was in a dust storm! Also, in the 1990s, it was hypostulated that dust from regions upwind of Israel helped clouds to rain on the southern margins of storms where dust was more likely to be encountered. Finally, during a field project in Saudi Arabia a few years ago, I also experienced an effect of a large particle dust episode on Stratocumulus clouds first hand. Surprisingly shallow Sc clouds contained drizzle drops and later, ice at high cloud top temperatures, higher than -10 C. The characteristic of these dust particles, apparently, was to produce a broad droplet spectrum below the freezing level (one that extends the droplet sizes past 30 um in diameter) and that in turn, accelerate the formation of ice and snow, which melts into rain. Also, it may be, too, that the dust particles are active as ice nuclei at high temperatures, science speak for triggering the formation of ice at temperatures warmer than about -12 C.
The point of this is that it is POSSIBLE that dust contributed yesterday to ice in shallow clouds, too.